The rotor or the stator is, as is generally known, composed of a magnetic mass in which permanent magnets are inserted. These permanent magnets may be disposed at the surface, directly facing the air gap, or, in a variant, may be disposed within the magnetic mass, then being said to be “buried”.
In order to optimize the cost and the performance of electric machines, it may be necessary to increase the number of magnets, notably when it is not possible to improve the quality thereof or to maintain the same performance with magnets of lesser quality.
Optimal electromagnetic performance is obtained when a buried magnet is in perfect contact, over each of its two polar faces, with the magnetic mass in which it is inserted. The passage of the magnetic flux of the magnets toward the magnetic mass is thus optimized. The term “polar face” of a magnet means a face of the magnet that is magnetized with an identical polarity, thus forming the north or south pole of the magnet.
However, there is generally play between the magnets and the magnetic mass in which the magnets are inserted, thus constituting an air gap from a magnetic viewpoint, which inevitably leads to losses in the electromagnetic performance of the machine. Play of this type may be related to the manufacturing constraints, which, for a reasonable cost, do not make it possible to observe very precise dimensions when cutting the magnetic mass or when manufacturing magnets. Play may also be present because, due to the fact that magnets are sensitive to corrosion, it may be necessary to cover them with a protective coating, which also leads to uncertainty regarding the dimensions of the magnets.
In addition, the installation constraints mean that a certain play must be preserved between the magnets and the magnetic mass so as to facilitate the insertion of the magnets in said mass, notably when the magnetic mass is formed of a stack of thin magnetic sheets. In fact, in this case, the walls of the magnetic mass might not be perfectly straight, taking into account the fact that they are formed of a stack of thin sheets, which may require even greater installation play. As is conventional, a tolerance range of 0.2 mm in the dimensions of the magnets may be provided, such that it may be necessary to provide minimal play of 0.1 mm on ether side of the magnets for the installation, besides the tolerance due to the design of the magnetic mass. Lastly, as is conventional, play of approximately 0.25 mm per magnet may be obtained, for example. The air gaps between rotor and stator currently used in electric machines of low to medium power may be approximately from 0.5 mm to 1 mm.
In the case in which the machine has a plurality of magnets disposed in a number of rows per pole in the magnetic mass, the play of magnets in different rows may be added together, thus lowering the magnetic performance of the machine further still.
There is thus a need to minimize the play between the magnets and the magnetic mass receiving the magnets so as to improve the magnetic performance of rotating electric machines.